Introduce the concept of a meta-package to PurlDB

Suppose that you want to "watch" a package to be sure that new versions of that package get populated in your PurlDB. In order to drive such a process (and others to be determined) you need the concept of a `meta-package`, a non-instantiated package without any version, that is more like a template than an actual package, so that you can indicate that you want to watch it, and configure the watch frequency and similar things. You could also keep the meta-package updated so that it always shows the very latest attribute values of that package, although that is probably a "stretch goal".

One simple way to identify a meta-package would be to use the reserved keyword `meta` in the Version of the Package URL. So you would not be able to derive a Download URL for such an entry, but you might be able to find its "home" URL, which would support any process that looks for new versions.

Just a few ideas. More details needed of course! Note that any client systems of the PurlDB would need to be aware that a meta-package is a special concept; perhaps this can be supported by enhancements to the APIs.